Using this service, having mail forwarded to a nonstandard port. What is the best way to lockdown mail server to only accept the mail being forwarded and rejecting mail from other servers?

whitelist DYNU.com servers to be only ones allowed to receive mail from?

To make sure our email server only accepts mail from DYNU and nothing else, add these mx addresses or the IPs they resolve to Whitelist IPs on our server??

Primary: mx1.dynu.com 207.38.69.195
Secondary: mx2.dynu.com 107.161.27.201